Molecular beam epitaxy (MBE) is a well known and widely used technique for building semiconductor (or other) materials on a substrate. Typically, MBE directs neutral beams of molecules at a target substrate, while maintaining process conditions (e.g., substrate temperature, etc.) and stoichiometry that encourage growth of the desired material (typically, a semiconductor crystal, such as GaAs) on the target substrate. A wide variety of useful semiconductor devices can be fabricated, in whole or in part, using MBE processes.
It is known in the art that, for many MBE-grown materials and devices fabricated from them, doping during the MBE process can produce performance advantages. Generally speaking, the greater the doping during the MBE process, the greater the realized performance advantages for many high-speed devices.
Also well known in the art is the fact that for various host materials (also referred to herein as matrix materials) under given growth conditions, there will be a saturation limit for the carrier concentration produced by a dopant, and attempts to increase the dopant concentration beyond this limit will generally fail to raise the maximum practical carrier concentration in the matrix material, or even without apparent carrier saturation may produce counterproductive side effects. These side effects include degraded minority carrier lifetimes and enhanced dopant diffusion, which often limit device performance and reliability. Typically, a given dopant's carrier saturation limit is lower than the dopant's actual solubility in a particular matrix material. Table 1 below provides exemplary saturation limits and solubilities for several commonly used dopant/matrix material combinations.
TABLE 1Bulk Carrier SaturationBulk Crystal SolubilityMatrixDopantLimitLimitGaAsBe2 − 4 × 1019 cm−32 × 1020cm−3GaAsC1 − 2 × 1020 cm−35 × 1019cm−3GaAsSi     1 × 1019 cm−32 × 1020cm−3GaAsMg     1 × 1020 cm−3>1020cm−3InPBe     4 × 1018 cm−3about 1020cm−3
Prior art methods cannot effectively dope near or beyond the carrier saturation limit because dopant concentrations near or beyond the carrier saturation limit behave differently in the crystal lattice compared to the dopant at lower concentrations. For example, when a GaAs matrix is doped with a Group II p-type dopant such as Be, the Be atoms generally occupy Ga sites (i.e., substitute for Ga atoms) until the dopant approaches the critical carrier saturation limit. In general, at doping levels above the critical carrier saturation limit, approximately half of the incremental Be dopant ions reside on interstitial sites in the host matrix where they become donors, compensating for the incremental half occupying Ga sites as acceptors. For Be and other dopants, additional compensation mechanisms may be present, such as crystalline defects, including anti-site and vacancy defects, dopant precipitation, amphoteric dopant compensation, rapid diffusion of the dopant to lower doped regions, and enhanced incorporation of compensating background impurities such as oxygen.
These compensating mechanisms generally tend to result in an increase in the non-radiative trap density. Minority carrier lifetime is reduced in the presence of such non-radiative traps, resulting in the reduction of the efficiency of minority carrier devices. Thus, in a material doped beyond carrier saturation, minority carriers typically have a much higher probability of recombining non-radiatively than materials doped below carrier saturation. This increase in non-radiative recombination probability greatly diminishes the electroluminescence or photoluminescence associated with radiative materials, degrading the efficiency of light-emitting diodes (LEDs) or stimulated-gain devices, such as photo-pumped emitters, superluminescent diodes or lasers. Shortened minority carrier lifetimes also diminish bipolar transistor gain, β. Non-radiative recombination has also been linked to performance degradation over time, leading to reliability concerns. For example, interstitial atoms provide both a non-radiative recombination site and tend to diffuse rapidly at elevated temperatures or in the presence of high-electrical fields, aging the material and causing performance degradation and failures. For example, interstitial Be dopant atoms in the base of a npn heterojunction bipolar transistor (HBT) tend to rapidly diffuse into the emitter, especially at elevated temperatures and under current stressing, irreversibly degrading the performance of the HBT.
The limitations observed for Be-doped GaAs have motivated those ordinarily skilled in the art to seek better p-type dopant alternatives. One such alternative dopant is carbon (C). C has been shown to be capable of doping GaAs p-type to a hole concentration in excess of 1020 cm−3 (see T. J. de Lyon, J. M. Woodall, M. S. Goorsky, and P. D. Kirchner, “Lattice-Contraction due to Carbon Doping of GaAs Grown by Metalorganic Molecular Beam Epitaxy.” Appl. Phys. Lett., 56, 1990, p. 1040). However, such heavily C-doped GaAs appears to exhibit a significant degradation in minority carrier lifetime (See R. K. Ahrenkiel, R. Ellingson, W. Metzger, D. I. Lubyshev, and W. K. Liu, “Auger recombination in heavily carbon-doped GaAs,” Appl. Phys. Lett., 78, 13, pp. 1879–81.), and experiments with heavily C-doped LEDs demonstrated a high cutoff frequencies but unacceptably low internal (hence, low external) quantum efficiencies for light-emitting applications, indicating a serious increase in the non-radiative recombination rate. (See T. J. de Lyon, J. M. Woodall, D. T. McInturff, R. J. S. Bates, J. A. Kash, P. D. Kirchner, and F. Cardone, “Doping Concentration Dependence of Radiance and Optical Modulation Bandwidth in Carbon-doped Ga0.51In0.49P/GaAs light-emitting Diodes Grown by Gas Source MBE.” Appl. Phys. Lett., 60, 1992, p. 353). Nonetheless, C is rapidly becoming the p-type dopant of choice for the base region of commercial GaAlAs/GaAs HBTs, because C atoms predominantly substitute for As atoms in the GaAs crystal instead of occupying interstitial or Ga sites, up to and even beyond the solubility limit of carbon (C) in GaAs (see Table 1). Be dopant atoms, since they become interstitial much more readily than C dopant atoms, therefore migrate much more readily than C, and are comparatively unstable.
There are, however, critical limitations for C doping. In GaAs, the equilibrium solubility limit for C doping is only about 5×1019 cm−3. Doping C to a concentration above this amount can lead to formation of precipitates, which are known to degrade HBT performance. (See Q. Yang, D. Scott, J. Miller, P. Meyer, H. C. Kuo, J. E. Baker, and G. E. Stillman, “Precipitate formation in carbon-doped base of InGaP/GaAs heterojunction bipolar transistors grown by low-pressure MOCVD.” Appl. Phys. Lett., 74, 20, pp. 2993–5.)
Recombination may occur via parallel channels associated with different recombination mechanisms. For example, if a material has only a radiative recombination channel then only light is generated. However, if a material has in addition a non-radiative recombination channel (with a higher recombination rate), then non-radiative recombination will be dominant. Non-radiative recombination greatly degrades electroluminescence, as well as limiting HBT transistor gain to unacceptably low values. Additionally, p-type C doping is technologically more difficult for In-based III-V compounds and their alloys, e.g., InP and InGaAs. Therefore, there is a significant need in the field for an alternative to C doping.